The metabolism of chlordimeform following intraperitoneal treatment of mice and oral treatment of rats and following incubation with rat liver microsomes was examined. In addition to chlordimeform and other previously identified metabolites the following novel metabolites were present: N'-(4-chloro-o-tolyl)formamidine, 1,1-dimethyl-3-(4-chloro-o-tolyl) urea, 1-methyl-3-(4-chloro-o-tolyl)urea, and 3-(4-chloro-o-tolyl)urea. Metabolism studies of chlordimeform in twospotted spider mites and in third and fifth instar cabbage looper larvae indicated a rapid uptake of the compound and high internal levels of radioactive material. Chlordimeform metabolism proceeded at a slower rate in susceptible mites and third instar cabbage looper larvae than in moderately tolerant fifth instar cabbage looper larvae. Differential metabolism likely plays a role in chlordimeform selectivity. Structure-toxicity relationship studies of 104 formamidines and spider mites revealed that for maximum toxicity formamidines must possess an aryl moiety with two substituents, and these two substituents should be at ring positions two and four. The following ring substituents yielded highly active formamidines: Cl-4, CH3-2; Cl-2,4; Cl-2, CH3-4; Br-4, CH3-2; F-2, CH3-4; CH3-2,4; and F-2,4. With regard to requirements on the amino nitrogen it was concluded that at least one substituent should be lower alkyl, preferably methyl, while greater flexibility existed relative to the other amino nitrogen substituent.